Generating a jittered digital signal using a serializer device

ABSTRACT

A serializer device is used for generation, from a parallel digital signal, of a clock signal or a serial binary data signal having a pre-determined amount of jitter. A binary number having consecutive groups of ones and zeroes, when serialized by the serializer device, produces a clock signal. By varying the number of ones and zeroes on the binary number, a pre-determined amount of jitter can be generated. Use of sigma-delta modulation in combination with a phase-locked loop circuitry allows one to obtain a smoothly varying jitter of the output signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims priority from U.S. patent application No.61/291,260 filed Dec. 30, 2009, entitled “Clock jitter generation usingSERDES” which is incorporated herein by reference for all purposes.

TECHNICAL FIELD

The present invention relates to testing digital communications systems,and in particular to the generation of jittered digital signals fortesting and troubleshooting of digital communications systems.

BACKGROUND OF THE INVENTION

In digital communications, information is transferred between twoterminals in the form of binary numbers. The terminals can be twoelectronic devices, such as a personal computer and a printer. Theinformation can be transferred over a parallel bus, such as in aparallel output port of the personal computer, or over a serialconnection, such as a Universal Serial Bus (USB) connection, which isnow widespread.

The serial digital information transfer is particularly important fordigital communications, because it can serve as an interface betweendigital systems employing different bit widths and internalcommunication protocols. Nowadays, disk drives in a personal computerfrequently use a serial, not a parallel, connection to the mainframeboard. This serial connection can be very fast due to high-speed serialcommunication enabled by serializer-deserializer (SERDES) integratedcircuits. Furthermore, serial digital communication also allows one touse a single carrier (such as a coaxial cable or an optical fiber) fordigital information transfer over large distances.

In serial digital communications, the information-carrying signal can beprovided with or without a separate clock signal. In systems without theseparate clock signal, the receiving circuitry generates an internalclock signal based on transitions between “one” and “zero” levels in thereceived serial digital signal itself. When the serial digital signal isnot very stable in bit frequency or phase, the receiving circuitry mayhave a difficulty in correctly establishing the internal clock signal.Similarly, when a timing variation is present between the clock and thedata signals in systems employing a separate clock signal, the receivingcircuitry may have a difficulty in interpreting the received bits asones or zeroes.

The timing variation of the digital binary signal, either with respectto a clock signal, or with respect to an “ideal” stable signal, is knownas jitter. Jitter is commonly present in digital communications systems.Stability with respect to jitter is a very important characteristic ofany electronic component or module employing serial or parallel digitalcommunications.

Various standards with respect to tolerable levels of jitter have beendeveloped. The digital communication components and modules need to betested against these jitter standards. To test stability of electroniccomponents with respect to jitter, a jittered clock reference signal,having a pre-defined amount of jitter, is required.

Cranford et al. in U.S. Pat. No. 7,512,177, incorporated herein byreference, discloses a jittered signal generator including a pulsegenerator coupled to a phase shifter. The phase shifter includes aplurality of quarter-wavelength lines having electrically variablecapacitors. The capacitors are controlled by an arbitrary waveformgenerator. In operation, the arbitrary waveform generator causes thephase shifter to introduce a pre-defined, time-dependent amount of phaseshift into the signal generated by the pulse generator, which introducesa pre-determined amount of jitter.

Frisch in U.S. Pat. No. 7,171,601, incorporated herein by reference,discloses a jitter generator based on a controllable delay line.Referring to FIG. 1, a jitter generator 10 includes a multiplexer 12, aprogrammable delay circuit 14, a programmable pattern generator 16, anda delay measurement unit 18. In a normal operating mode of the jittergenerator 10, the multiplexer 12 delivers an input (“VIN”) signal to thedelay circuit 14, and the delay circuit 14 delays the VIN signal byvarying amounts of time to produce a jittery “TEST” signal. Clocked by ajitter clock signal JCLK, the pattern generator 16 supplies a sequenceof digital “DELAY” words to the delay circuit 14, and each successiveDELAY word controls a delay of the delay circuit 14. With the patterngenerator 16 programmed to produce a suitable DELAY data sequence, thejitter generator 10 can produce a jittery TEST signal having a varietyof jitter frequencies and amplitudes.

Jittered signal generators of the prior art commonly suffer from adrawback of complexity. By way of example, the multiplexor 12, theprogrammable delay circuit 14, the programmable pattern generator 16,and a delay measurement unit 18 of the prior-art jitter generator 10 ofFIG. 1 can occupy a significant area on a printed circuit board.

The prior art is lacking a jittered signal generator that would becompact, simple, and inexpensive to implement. It is desirable that ajittered signal generator have a small number of electronic modules, forexample two or even one module. The present invention provides such asolution.

SUMMARY OF THE INVENTION

In the present invention, a serializer device is used for generation,from a parallel digital signal, of a clock signal or a serial binarydata signal having a pre-determined amount of jitter. Modern fieldprogrammable gate arrays (FPGA) implemented serializer-deserializer(SERDES) devices, while being relatively inexpensive, can operate atspeeds of a few gigabit per second. A simple source of a jittered serialdigital signal at speeds of a few hundred megabit per second can beeasily constructed using a SERDES device.

In accordance with the invention, there is provided a jittered signalgenerator for generating a jittered digital signal having first andsecond signal levels, comprising:

a binary number generator for generating a first binary number havingadjacent first and second groups of bits of all ones and all zeroes,respectively; and

a serializer device coupled to the binary number generator, forserializing the first binary number forming a serially outputted groupof bits of all ones and a serially outputted group of bits of allzeroes, so as to obtain the jittered digital signal,

wherein the serially outputted group of bits of all ones corresponds tothe first signal level, and the serially outputted group of bits of allzeroes corresponds to the second signal level of the jittered digitalsignal.

In accordance with another aspect of the invention, there is furtherprovided a method for generating a jittered digital signal having firstand second signal levels, the method comprising:

-   (a) generating a first binary number having adjacent first and    second groups of bits of all ones and all zeroes, respectively; and-   (b) serializing the first binary number using a serializer device    forming a serially outputted group of bits of all ones and a    serially outputted group of bits of all zeroes, so as to obtain the    jittered digital signal,    wherein the serially outputted group of bits of all ones corresponds    to the first signal level, and the serially outputted group of bits    of all zeroes corresponds to the second signal level of the jittered    digital signal.

The jittered digital signal can be obtained by providing to theserializer more than two groups of ones and zeroes of different size,and/or by varying the number of bits in the first group of all ones, inthe second group of all zeroes, or in both groups. The number of bitscan be varied by cyclically shifting the first binary number using abarrel-type bit shifter. The barrel shifter is digitally controlled by agenerator of a desired jitter pattern. A sigma-delta shifter coupled tothe barrel-shifter, used in conjunction with a phase-locked feedbackloop (PLL) coupled to an output of the serializer device, can beemployed to produce a smoothly varying output jitter pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments will now be described in conjunction with thedrawings, in which:

FIG. 1 is a block diagram of a prior-art generator of a jittered signal;

FIG. 2 is a prior-art timing diagram showing an ideal clock signal and aclock signal having a sinusoidal jitter pattern;

FIG. 3 is a prior-art block diagram of a high-speed serial linkemploying a pair of serializer-deserializer (SERDES) devices;

FIGS. 4A and 4B are timing diagrams illustrating generation of ajittered signal using a SERDES device, according to the invention;

FIG. 5 is a block diagram of a jittered signal generator of theinvention, illustrating SERDES operation for generating a clock signal;

FIG. 6 is a block diagram of a jittered signal generator of FIG. 5,illustrating generation of a controlled amount of jitter using a barrelshifter coupled to the SERDES of FIG. 5,

FIG. 7 is a time (or pulse number) dependence of a magnitude of a jittergenerated by the jittered signal generator of FIG. 6;

FIG. 8 is a block diagram of a jittered signal generator equipped with asigma-delta shifter;

FIG. 9 is a block diagram of a phase-locked feedback loop (PLL) circuitfor smoothing the phase variation introduced by the sigma-delta shifterof FIG. 8, shown together with time traces of jitter before and afterthe smoothing;

FIG. 10 is superimposed time traces of jitter generated by the jitteredsignal generator of FIG. 8, before and after smoothing by the PLLcircuit of FIG. 9;

FIG. 11 is a block diagram of a barrel shifter controller for use in thejittered signal generators of FIGS. 6 and 8;

FIG. 12 is a block diagram of a binary number generator including aplurality of registers;

FIG. 13 is a block diagram of the binary number generator of FIG. 12,including a plurality of barrel shifters and sigma-delta shifters;

FIG. 14 is a block diagram of a control circuit for the plurality of thesigma-delta shifters;

FIG. 15 is a block diagram of a barrel shifter controller for use in thebinary number generator of FIG. 13; and

FIGS. 16 to 18 are block diagrams of a preferred implementation of ajittered signal generator of the invention, including a fieldprogrammable gate array (FPGA)-implemented SERDES.

DETAILED DESCRIPTION OF THE INVENTION

While the present teachings are described in conjunction with variousembodiments and examples, it is not intended that the present teachingsbe limited to such embodiments. On the contrary, the present teachingsencompass various alternatives, modifications and equivalents, as willbe appreciated by those of skill in the art.

Referring to FIG. 2, an ideal clock signal 20 includes a train of pulses21 having raising edges 27 disposed at equal time intervals T from eachother. In a jittered clock signal 22, the raising edges 27 of the pulses21 are disposed at generally uneven time intervals. The jitter, or thetemporal displacement of the raising edges 27 of the pulses 21 of thejittered clock signal 22 from ideal positions 23, is plotted at 24. Thejitter 24 has a sinusoidal envelope 25. The tolerance of a digitaldevice to jitter is frequently defined using maximum tolerable amplitudeof the sinusoidal jitter 25.

Turning to FIG. 3, a typical high-speed serial link 30 between terminals31 and 32 includes two serializer-deserializer (SERDES) devices 33, onein the terminal 31 and one in the terminal 32. Two SERDES devices 33establish a high-speed serial interface 34 between the terminals 31 and32 having identical or different parallel interfaces 35 and 36,respectively. The high-speed serial link 30 is a typical application forthe SERDES devices 33.

Referring now to FIGS. 4A and 4B, an application of a serializer unit 37of the SERDES device 33 for jitter generation according to the inventionis illustrated by means of timing diagrams 40A and 40B, respectively.The timing diagram 40A of FIG. 4A illustrates a process of generation ofan ideal clock signal 41A by the 8-bit serializer unit 37 of the SERDES33. To generate the ideal clock signal 41A, a binary number 42 includingadjacent groups of four “ones” and four “zeros”, “11110000”, is appliedto the serializer unit 37. The serializer unit 37 generates the idealclock signal 41A in form of symmetrical pulses at a fixed frequency. Theserially outputted group of bits of all ones commonly corresponds to ahigh signal level, and the serially outputted group of bits of allzeroes commonly corresponds to a low signal level of the output pulses.In the example given, the frequency of the ideal clock signal 41A isone-eighth of the bit rate of the SERDES 33. For example, if the SERDES33 operates at the frequency of 4.97664 GHz, the ideal clock signal willhave a frequency of 622.08 MHz.

The timing diagram 40B of FIG. 4B illustrates a process of generation ofa jittered clock signal 41B by the 8-bit serializer unit 37. In thetiming diagram 40B, binary numbers 43, 44, and 45 are applied to theserializer unit 37, one after the other. The binary number 43 includesadjacent groups of four “ones”, three “zeroes”, and one “one”,“11110001”. The binary number 44 includes adjacent groups of three“ones”, three “zeroes”, and two “ones”, “11100011”. The binary number 45includes adjacent groups of three “ones”, four “zeroes”, and one “one”,“11100001”. When these numbers are inputted one by one into theserializer unit 37, the output signal 41B of the serializer unit 37 willno longer be symmetrical pulses at a fixed frequency. The output signal41B will be jittered. The magnitude of jitter, δT, is shown at 46.

Turning to FIG. 5, use of the SERDES 33 for generating the clock signal41A is further illustrated by means of a block diagram of a jitteredsignal generator 50 including a SERDES input register 52 and the SERDESdevice 33. The SERDES device 33 includes an optional ×8 Clock MultiplierUnit (CMU) 51. The SERDES input register 52, containing the binarynumber 42, including adjacent groups of four “ones” and four “zeros”,“11110000”, is coupled to the serializer unit 37. In operation, a stablereference signal 53 at the frequency of 622.08 MHz is applied to the CMU51. The CMU 51 produces an output signal 54 at a frequency of eighttimes the frequency of the reference signal 53, that is, 4.976 GHz. Theoutput signal 54 is used to clock the serializer unit 37 of the SERDES33. The serializer unit 37 outputs the clock signal 41A at the frequencyof 622.08 MHz. As shown in FIG. 4B, changing the input binary numberstored in the SERDES input register 52, that is, sequentially storingthe numbers “1111001”, “11100011”, “11100001”, . . . instead of thenumber “11110000”, will result in the output signal of the SERDES 33being jittered in a controllable fashion.

As is understood by one skilled in the art, the frequency of operationand the number of bits in a serialized word of the jittered signalgenerator 50 may be different from the ones disclosed herein. Also, theSERDES input register 52 can contain a binary number having unequalnumbers of ones and zeroes. In general, to obtain an output signal usingthe SERDES 33, the following steps need to be taken:

-   (a) generating a number including adjacent first and second groups    of bits of all ones and all zeroes, respectively, and-   (b) serializing the generated number using the serializer unit 37,    forming a serially outputted group of bits of all ones and a    serially outputted group of bits of all zeroes.

The serially outputted group of bits of all ones will correspond to onesignal level (for example, “high” signal level), and the seriallyoutputted group of bits of all zeroes will correspond to the othersignal level (for example, “low” signal level).

To jitter the output signal, the number generated in step (a) can haveunequal numbers of ones and zeroes; and/or the number generated in step(a) can have more than two groups of ones and zeroes. Alternatively orin addition, the steps (a) and (b) can be repeated, while changing thetotal number of bits in at least one of the first group and the secondgroup of the number of step (a). According to the invention, thechanging of the total number of bits in the first in the second groupsof bits is preferably done using a cyclic shifting of an initial number.An apparatus for achieving such cyclic shifting will now be described.

Referring to FIG. 6, a jittered signal generator 60 of the inventionincludes a 3-bit barrel shifter 61, a jitter phase register 62, aninitial register 63, the SERDES 33 having the serializer unit 37, andthe SERDES input register 52. In operation, three bits of a numberstored in the jitter phase register 62 are used as a jitter controlsignal, to control the barrel shifter 61 via a 3-bit control bus 64. Thefunction of the barrel shifter is to cyclically shift a number (forexample, “11110000”) stored in the initial register 63. The cyclicalshift is shown by arrows 65 in FIG. 6. Then, the shifted number isplaced into the SERDES input register 52, for serialization by theserializer unit 37. Thus, a pre-determined amount of jitter in an outputsignal 41C can be generated by the jittered signal generator 60 bycontrolling the barrel shifter 61 through the control bus 64. Forexample, in FIG. 6, a control binary number “011” is applied to thecontrol bus 64. This causes the barrel shifter 61 to shift a binarynumber stored in the initial register 63 (in the example shown, thenumber “11110000”), by three 1-bit positions, because the binary “011”is three. Since the serializer unit 37 operates at a frequency ofapproximately 5 GHz, shifting by three 1-bit positions causes the outputsignal to be jittered by three (⅕ GHz), or three 200-ps time intervals.When the control binary number is changed, the magnitude of the cyclicalbit shift introduced by the barrel shifter 61 changes; and the magnitudeof the generated jitter changes accordingly. The “step size” of thejitter change is, in this case, 200 ps. The least significant bits ofthe jitter phase register 62 are used for sigma-delta modulation, whichwill be described in detail further below.

The time dependence of jitter in the output signal 41C is illustrated inFIG. 7, where the magnitude of the jitter is plotted as a function of apulse number in a pulse sequence of the output signal 41C. The verticalscale is in Unit Intervals (UI), which is the time between bits in ajitter free bit sequence. A stepped trace 71 represents the jitter ofthe output signal 41C. It is seen that the jitter changes in 200-psintervals (0.03 UI). A “smoothed” trace 73 represents the jitter thathas been smoothed by using a phase-locked feedback loop (PLL) circuit,not shown. An embodiment of a jittered signal generator having a PLLwill be described further below.

According to the present invention, sigma-delta modulation can be usedto dither the steps in the jitter seen in the unsmoothed trace 71.Sigma-delta modulation, when used together with a PLL circuit, canproduce an output signal that is significantly closer to a pure sinusoidthan the “smoothed” trace 73 of FIG. 7.

Referring now to FIG. 8, a jittered signal generator 80 has the SERDES33 including the serializer unit 37 and the CMU 51 for clocking theserializer unit 37 at the high frequency of 4.976 GHz, the barrelshifter 61, the jitter phase register 62 for controlling the barrelshifter 61 via the 3-bit control bus 64, the initial register 63 forproviding an initial number to the barrel shifter 61, an intermediateregister 81 for storing a number that has been cyclically shifted by thebarrel shifter 61, a <<sigma-delta>> cyclical shifter 82, a SERDES inputregister 83, a <<sigma-delta>> register (n-bit accumulator) 84 forcontrolling the timing of the cyclical shift by the sigma-delta shifter82 through a <<signma-delta>> control line 85, a transition detector 88,and a summing module 86.

The operation of the jittered signal generator 80 is similar to that ofthe jittered signal generator 60 of FIG. 6. One important difference isthat one more cyclical shifter, specifically, the sigma-delta shifter82, is used to additionally shift the bits of the binary number alreadyshifted by the 3-bit barrel shifter 61. The sigma-delta shifter 82 is asingle-bit shifter. It is controlled by the sigma-delta register 84 andthe transition detector 88, whose function is to couple the mostsignificant bit (MSB) of the sigma-delta register 84 to the sigma-deltashifter 82. The summing module 86 functions to add the numberrepresented by the least-significant bits (LSBs) of the number stored inthe jitter phase register 62, to a previously obtained number. In otherwords, the sigma-delta register 84 accumulates least significant bits ofthe jitter phase register 62. The LSBs of the jitter phase register 62indicate the proximity of a next 200-ps increment. This arrangementallows the phase of an output signal 41D to be dithered prior to, and inanticipation of, a next 200-ps increment, providing a first-ordersigma-delta modulation of the phase of the output signal 41D.

Turning to FIG. 9, a plot 91 represents a time dependence (or a pulsenumber dependence, because the pulses are quasi-periodical) of jitter ofthe output signal 41D. The sigma-delta modulation results in ahigh-frequency ripple 92 in the jitter function 91. The function of aphase-locked feedback loop (PLL) circuit 90 is to remove thehigh-frequency ripple 92, so as to reveal a smooth sine dependence ofthe jitter. A plot 93 shows the time dependence of jitter of the outputsignal 41D with the high-frequency ripple 92 removed. The PLL circuit 90shown in FIG. 9 includes serially coupled phase detector 94, a low-passloop filter 95, and to a voltage-controlled oscillator (VCO) 96. Inoperation, the phase detector 94 detects a phase difference betweenincoming and output signals 97 and 98, respectively; the low-pass loopfilter 95 filters out high-frequency components; and the VCO 96recreates the clock signal with a smoothly varying phase. The PLLcircuit 90 thus functions as a low-pass filter, or a smoothing filter,for the phase of the output signal 41D.

Turning to FIG. 10, the plots 91 and 93 of FIG. 9 are overlapped tohighlight the smoothing by the PLL 90. A pulse number trace 101corresponds to the plot 91 of FIG. 9, and a pulse number trace 103corresponds to the plot 93 of FIG. 9. The output phase, plotted at 103,is much closer to a pure sinusoid than it is in the smoothed trace 73 ofFIG. 7, even though the same PLL circuit 90 has been used to obtain thephase variation 73 of FIG. 7. The improved performance of thesigma-delta modulation technique is a result of a high-frequencycharacter of the phase modulation brought forward by the sigma-deltamodulation. The high-frequency modulation can be filtered out moreefficiently than a simple stepwise modulation shown in the stepped trace71 of FIG. 7.

As noted above, the magnitude of the jitter in the output signal of thejitter generators 60 and 80 shown in FIGS. 6 and 8, respectively, isdetermined by a number stored in the phase register 62, which is coupledthrough the control bus 64 to the barrel shifter 61. In other words, thenumber stored in the jitter phase register 62 serves as a control signaldetermining the magnitude of the jitter. By generating a sequence ofsuch numbers and placing them one-by-one in the jitter phase register62, a jittered signal having a desired time dependence of jitter can begenerated.

Referring to FIG. 11, a barrel shifter controller 110 is used togenerate numbers to be placed into the jitter phase register 62. Thebarrel shifter controller 110 includes a ramp signal generator 111, asine lookup RAM 112, and an optional output multiplier 113. The rampsignal generator 111 includes a phase increment register 114, anaccumulator register 115, and a summation module 115A. The outputmultiplier 113 includes a multiplication unit 116 and an amplitudemodifier register 117. In operation, the barrel shifter controller 110is clocked at 622.08 MHz. A phase increment value stored in the phaseincrement register 114 is added to a running sum of the phase incrementvalues, so as to generate a linearly increasing ramp signal 118. Theramp values are used as addresses of the memory cells of the sine lookupRAM 112, which provides sine values shown at 119A. The sine values aremultiplied by the multiplication unit 116 by a value stored in theamplitude modifier register 117. The output jitter control signal isshown at 119B. The output jitter control signal 119B has a sinusoidalshape. Referring back to FIG. 8, the jitter control signal 119B isplaced into the jitter phase register 62. Three bits of the jitter phaseregister 62, coupled to the three-bit bus 64, are used to control thebarrel shifter 611; the least significant bits (LSBs) are used forsigma-delta modulation; and the most significant bits (MSBs) are notused in this embodiment. The number of bits used and not used, theclocking frequency, the type of memory, etc., may of course vary.

The jittered signal generators 50, 60, and 80 are clocked at 622.08 MHz.For some FPGA circuits, this clocking frequency may be too high.According to the present invention, the clocking frequency may beeffectively lowered by increasing the bit number per a singularserialization step. To handle the increased bit numbers withoutincreasing the registers bit size, multiple registers can be used tomake up for the increased bit number. An example embodiment of ajittered signal generator with “concatenated” registers will now bedisclosed.

Referring to FIG. 12, a jittered signal generator 120 includes a 32-bitSERDES device 123 and four 8-bit input registers 122A to 122D coupledthereto, each assigned 8 bits of the 32 input bits of a serializerdevice 127. The serializer device 127 is a part of the 32-bit SERDESdevice 123. The SERDES device 123 includes an optional x32 ClockMultiplier Unit (CMU) 121. In operation, a stable reference signal 129at the frequency of 155.52 MHz is applied to the CMU 121. The CMU 121produces an output signal 124 at the frequency of 32 times the frequencyof the reference signal 129, that is, 4.976 GHz. The 8-bit registers122A to 122D are loaded with the same digital number 42, “11110000”.Each of the 8-bit registers 122A to 122D is clocked at 155.52 MHz, whilean output serialized signal 128 is at 622.08 MHz.

Referring back to FIG. 5, the jittered signal generator 50 of FIG. 5 isclocked at 622.08 MHz. Referring again to FIG. 12, the jittered signalgenerator 120 is clocked at 155.52 MHz. Thus, the clock frequency isreduced 4 times, that is, the global clock frequency is reduced by thenumber of the 8-bit registers 122A to 122D of the jittered signalgenerator 120.

Referring now to FIG. 13, a jittered signal generator 130 includes fourinitial binary number registers 133A to 133D, each for storing aninitial binary number having two adjacent groups of bits of all ones andall zeroes; four three-bit barrel shifters 131A to 131D controlled byfour jitter phase outputs 135A to 135D via four 3-bit buses 134A to134D, for cyclically shifting the bits of an initial binary numberprovided by the four initial binary number registers 133A to 133D,respectively, so as to generate a plurality of intermediate binarynumbers; and four optional sigma-delta shifters 132A to 132D, forsigma-delta modulation of the corresponding intermediate numbers. Theoperation of the jittered signal generator 130 is similar to that of thejittered signal generator 80 of FIG. 8. One difference is that fourinitial registers 133A to 133D are used instead of the single initialregister 63; four 3-bit barrel shifters 131A to 131D are used instead ofthe single barrel shifter 61; and four sigma-delta shifters 132A to 132Dare used instead of the single sigma-delta shifter 82. Anotherdifference is that the internal clock frequency of the jittered signalgenerator 130 is four times less than the clock frequency of thejittered signal generator 80, due to using four registers in parallel,instead of just one register.

The following Table 1 illustrates the advancement of three jitter outputcontrol bits of the jitter phase outputs 135A to 135D, controlling thethree-bit barrel shifters 131A to 131D via the three-bit buses 134A to134D. The bits shown are for three successive clock positions at 155.52MHz clock frequency.

TABLE 1 Clock 1 Clock 2 Clock 3 Jitter phase output 010 010 011 135AJitter phase output 010 011 011 135B Jitter phase output 010 011 011135C Jitter phase output 010 011 011 135D

With regards to Table 1 above, in the jittered signal generator 60 ofFIG. 6 clocked at 622.08 MHz, the phase of the output signal 41C wouldhave changed on the 6^(th) clock. In the jittered signal generator 130of FIG. 3, the phase of the output signal 128 has changed in the secondbarrel shifter 131B controlled by the second jitter phase output 135B,on the second clock (Clock 2 in Table 1 above).

As noted above, sigma-delta shifters 132A to 132D are optional. However,their use, in combination with a PLL circuit similar to the PLL circuit90 of FIG. 9, will greatly reduce granularity of jitter. The reducedgranularity can be seen by comparing FIGS. 7 and 10, specifically bycomparing the plots 73 and 103 showing the jitter of output pulses atthe output of the PLL circuit 90. The number of registers used inparallel may vary; the choice depends on bit size of SERDES inputregister and the FPGA clock frequencies available. In general, the clockfrequency for clocking the jittered signal generator 130 is reduced ntimes, where n is the number of the registers used in parallel.

The sigma-delta shifters 132A to 132D of FIG. 13 are controlled byseparate sigma-delta registers. Referring to FIG. 14, four sigma-deltashifters 132A to 132D are controlled by four sigma-delta registers 144Ato 144D, respectively. The four sigma-delta registers 144A to 144D arecoupled to the four jitter phase outputs 135A to 135D, respectively.Four addition units 146A to 146C are used to accumulate the LSBs of thefour jitter phase outputs 135A to 135D as shown. Four transitiondetectors 148A to 148D couple the most significant bits of thesigma-delta registers 144A to 144D to the sigma-delta shifters 132A to132D, respectively. This sigma-delta modulation arrangement is similarto the arrangement for the sigma-delta modulation shown in FIG. 8. Itsfunction is to provide the sigma-delta modulation to all four eight-bitwords 138A to 138D, which are being serialized by the serializer unit127 of the SERDES 123 at a single clock period of the 155.52 MHz clocksignal 129.

Turning now to FIG. 15, a four-channel barrel shifter controller 150 isused to generate numbers to be placed in the jitter phase registers 135Ato 135D. The barrel shifter controller 150 includes a four-channel rampsignal generator 151, four sine lookup RAM units 152A to 152D, and anoptional four-channel output multiplier 153. The four-channel rampsignal generator 151 includes a phase increment register 154, anaccumulator register 155, four summation modules 155A to 155D, and three×2, ×3, and ×4 scaling modules 158B to 158D, respectively. Thefour-channel output multiplier 153 includes four multiplication units156A to 156D and an amplitude modifier register 157. The operation ofthe four-channel barrel shifter controller 150 is similar to theoperation of the single-channel barrel shifter controller 110 of FIG.11. The four-channel barrel shifter controller 150 produces the fourjitter phase outputs 135A to 135D. The function of the three ×2, ×3, and×4 scaling modules 158B to 158D is to phase-shift the four jitter phaseoutputs 135A to 135D relative to each other. The amplitude of the jitterphase outputs 135A to 135D is determined by the amplitude multiplier157.

It is to be understood that the number of registers used in parallel inthe barrel shifter controller 150 may vary. The clock frequency will bereduced (divided) by the number of registers used in parallel. Forexample, the clock frequency of the four-channel barrel shiftercontroller 150 is four times lower than that of the single-channelbarrel shift controller 110.

Hardware implementations of the jittered signal generator 130 of theinvention will now be considered. Turning to FIGS. 16 to 18, a jitteredsignal generator 160 is implemented using a FPGA Module 161 coupled to aPhysical Interface Module (PIM) 163, both of which are manufactured byJDS Uniphase Corporation having headquarters in California, USA. TheFPGA Module 161 includes an FPGA 162 having implemented therein a SERDESmodule 165. The PIM module 163 has no FPGA circuitry, but has severalplesiochronous digital hierarchy (PDH) physical interfaces, including aline interface unit (LIU) 164. Referring to FIGS. 17 and 18, a digitaljitter generator 171, a sigma-delta modulator 172, and the SERDES 165are implemented in the FPGA 162. The SERDES 165 corresponds to theSERDES 33 of FIGS. 5, 6, 8 and to the SERDES 123 of FIGS. 12 and 13.Turning to FIG. 18 specifically, the PLL 90 is implemented in PIM 163. Asmoothly jittered clock signal 181 can be used as an output jittereddigital signal, or, as shown in FIG. 18, it can be supplied to a datasource 182 to be used for generation by the LIU 164 of a jittered datasignal 183. The jittered clock signal 181 and/or the jittered datasignal 183 have a pre-determined amount of jitter and can be used fortesting electronic components and modules.

Other hardware implementations of the jittered signal generators 50, 60,80, 120, and 130 of the invention are possible, depending on particularserializer architecture used. As disclosed above, using a barrel shifterto cyclically shift the parallel signal is preferable to provide thepre-determined amount of jitter. Further, preferably, sigma-deltamodulation is used for modulating the cyclical shift produced by thebarrel shifter. A phase-locked loop can be used for smoothing thecyclical shift modulated using sigma-delta modulation. Other phasesmoothing arrangements can also be used.

The foregoing description of one or more embodiments of the inventionhas been presented for the purposes of illustration and description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed. Many modifications and variations are possiblein light of the above teaching. It is intended that the scope of theinvention be limited not by this detailed description, but rather by theclaims appended hereto.

1. A method for generating a jittered digital signal having first andsecond signal levels, the method comprising: (a) generating a firstbinary number including adjacent first and second groups of bits of allones and all zeroes, respectively; and (b) serializing the first binarynumber using a serializer device forming a serially outputted group ofbits of all ones and a serially outputted group of bits of all zeroes,so as to obtain the jittered digital signal, wherein the seriallyoutputted group of bits of all ones corresponds to the first signallevel, and the serially outputted group of bits of all zeroescorresponds to the second signal level of the jittered digital signal.2. The method of claim 1, further comprising (c) smoothing a phasevariation of the first binary number serialized in step (b), so as tosmooth a phase variation of the jittered digital signal.
 3. The methodof claim 2, wherein in step (c), the smoothing is achieved by coupling aphase-locked feedback loop to an output of the serializer device.
 4. Themethod of claim 1, comprising repeating steps (a) and (b), wherein inthe repeated step (a), the total number of bits in at least one of thefirst group and the second group is changed, whereby the digital signalis jittered.
 5. The method of claim 1, wherein step (a) includesproviding an initial binary number having adjacent first and secondgroups of bits of all ones and all zeroes, respectively; and cyclicallyshifting the bits of the initial binary number, so as to generate thefirst binary number.
 6. The method of claim 5, wherein the adjacent thefirst and the second groups of bits of the provided initial binarynumber have equal number of bits.
 7. A jittered signal generator forgenerating a jittered digital signal having first and second signallevels, comprising: a binary number generator for generating a firstbinary number including adjacent first and second groups of bits of allones and all zeroes, respectively; and a serializer device coupled tothe binary number generator, for serializing the first binary numberforming a serially outputted group of bits of all ones and a seriallyoutputted group of bits of all zeroes, so as to obtain the jittereddigital signal, wherein the serially outputted group of bits of all onescorresponds to the first signal level, and the serially outputted groupof bits of all zeroes corresponds to the second signal level of thejittered digital signal.
 8. The jittered signal generator of claim 7,further comprising a smoother for smoothing a phase variation of thejittered digital signal.
 9. The jittered signal generator of claim 8,wherein the smoother comprises a sigma-delta modulator and/or aphase-locked feedback loop.
 10. The jittered signal generator of claim7, wherein the binary number generator comprises: an initial binarynumber register for storing an initial binary number having adjacentfirst and second groups of bits of all ones and all zeroes,respectively; and a barrel shifter for cyclically shifting the bits ofthe initial binary number provided by the initial binary numberregister, so as to generate the first binary number.
 11. The jitteredsignal generator of claim 10, wherein the binary number generatorfurther comprises a barrel shifter controller for controlling the barrelshifter by means of a jitter control signal.
 12. The jittered signalgenerator of claim 11, wherein the barrel shifter controller comprises:a ramp signal generator for providing increasing ramp values; and amemory including a plurality of memory cells for storing a lookup tableof values of the jitter control signal, wherein the memory is coupled tothe ramp signal generator, so as to use the ramp values as addresses ofthe memory cells.
 13. The jittered signal generator of claim 12, whereinthe jitter control signal comprises a sinusoidal signal.
 14. Thejittered signal generator of claim 7, wherein the binary numbergenerator includes a plurality of initial binary number registers eachfor storing an initial binary number having adjacent first and secondgroups of bits of all ones and all zeroes, respectively; and a pluralityof barrel shifters, each such barrel shifter configured for cyclicallyshifting the bits of an initial binary number provided by one of theplurality of the initial binary number registers, so as to generate aplurality of intermediate binary numbers; wherein the plurality of thebarrel shifters are operationally coupled to the serializer device suchthat the plurality of the intermediate binary numbers are combinable toform the first binary number for serializing by the serializer device,whereby a clock frequency for clocking the jittered signal generator isreduced n times, wherein n is the number of the initial binary numberregisters in the plurality of the initial binary number registers. 15.The jittered signal generator of claim 14, wherein the binary numbergenerator further comprises a barrel shifter controller for controllingthe plurality of the barrel shifters by means of a plurality of jittercontrol signals, wherein the barrel shifter controller includes a rampsignal generator for providing increasing ramp values; and a pluralityof memory units, each such unit including a plurality of memory cellsfor storing a lookup table of values of the jitter control signal,wherein each of the plurality of the memory units is coupled to the rampsignal generator so as to use the ramp values as addresses of thecorresponding memory cells; wherein each of the plurality of the memoryunits is coupled to a particular one of the plurality of barrelshifters, for providing a corresponding one of the plurality of jittercontrol signals.
 16. Use of a serializer device for generation, from aparallel signal, of a clock signal or a serial binary data signal havinga pre-determined amount of jitter, wherein the serializer device isFPGA-implemented; wherein a barrel shifter is used to cyclically shiftthe parallel signal to provide the pre-determined amount of jitter; andwherein sigma-delta modulation is used for modulating the cyclical shiftproduced by the barrel shifter.
 17. The use of claim 16, wherein aphase-locked loop is used for smoothing the cyclical shift modulatedusing the sigma-delta modulation.